1. Field of the Invention
The present invention relates to a multibeam antenna used in a wireless communication system, more particularly to a multibeam antenna which includes auxiliary antenna elements disposed next to main antenna elements and to an antenna system in which such an antenna is used.
2. Description of Related Art
An antenna for forming multiple beams by supplying power thereto from a Butler-matrix feeder circuit is known hitherto. The Butler-matrix feeder is proposed and described in ELECTRONIC DESIGN, VOL. 9, pp. 170-173, issued April, 1961 under a title "Beam-forming matrix simplifies design of electronically scanned antennas." The Butler-matrix feeder has 2.sup.n input/output ports and composed of hybrid circuits and phase shifters both connected via transmission lines. 2.sup.n antenna elements are connected to output ports of the Butler-matrix feeder and constitute an antenna array which forms 2.sup.n beams.
FIG. 19 shows a multibeam antenna using the Butler-matrix feeder (referred to as a Butler-matrix antenna). A Butler-matrix feeder circuit 2 has input ports 1a-1d, and antenna elements 3a-3d constituting an antenna array are connected to the Butler-matrix feeder circuit 2. The Butler-matrix feeder circuit 2 is used for forming multiple beams each having a different radiation pattern, the number of which is equal to the number of input ports. The feeder circuit 2 includes first stage hybrid circuits 21a, 21b, constant-phase shifters 22a, 22b, and second stage hybrid circuits 23a, 23b. Electric power supplied from the input ports 1a-1d to the feeder circuit 2 is converted into outputs having a predetermined phase difference and is fed to four antenna elements 3a-3d which in turn form four beams to be transmitted to directions different from one another. The input ports 1a-1d function as output ports when outside radio waves are received by the antenna elements 3a-3d.
An example of a multibeam antenna circuit layout is shown in FIG. 20. In this example, micro-strip lines are used as transmission lines, and linearly polarized patch antennas are used as the antenna elements 3a-3d. The transmission lines are formed on both the front and rear surfaces of a three-layer substrate 4, and a ground plate is embedded in the substrate 4. The transmission lines on the front surface are connected to those on the rear surface via through-holes 24 formed on the substrate 4. More particularly, the first stage hybrid circuits 21a, 21b are formed on the front surface and the second stage hybrid circuits 22a, 22b on the rear surface. Each antenna element 3a-3d is connected to the second stage hybrid circuits 22a, 22b via a respective transmission line, and each second stage hybrid circuit 22a, 22b is connected to each first stage hybrid circuit 21a, 21b with respective two transmission lines as shown in FIG. 20 (solid lines are formed on the front surface and the dotted lines on the rear surface). There are eight connecting lines altogether, and one through-hole 24 corresponds to each connecting line. In this arrangement, phase difference due to the through holes 24 can be neglected.
Simulation results as to the circuit shown in FIG. 20 are shown in the graph of FIG. 21, assuming that a distance between neighboring two antenna elements is one half of the wavelength. The radiation pattern of four beams, i.e., relative power intensity versus beam angle, is plotted together with sidelobes. In the Butler-matrix antenna, the power fed from the input ports is distributed to the output ports with the same amplitude and a predetermined phase difference. Accordingly, the beam shapes formed by the antenna are solely determined depending on the distance among antenna elements, and formation of large sidelobes is unavoidable, as generally known. Moreover, the number of the antenna elements is limited to the numbers which are in units of 2.sup.n, i.e., 2, 4, 8, 16, etc. Therefore, it is difficult to arbitrarily increase the antenna gain.